1. Field of the Invention
The present invention relates to a laser and a method of stabilising a laser. The preferred embodiment relates to a method of stabilizing the energy of a pulsed SLM solid-state laser using a modulation technique.
2. Description of the Related Art
For certain holographic applications it is desirable to be able to use a red, green and blue (RGB) laser to write holographic pixels of a colour hologram. In order to ensure a good interference between the object and reference beams which are used to write the holographic pixels, the coherence length between the two beams should preferably be longer than the path difference of the two beams. As a result, in order to have a suitably long coherence length, it is highly desirable that the laser used in such applications should operate in a single longitudinal mode (SLM). In order to ensure that the laser operates in a SLM an etalon may be provided within the resonator cavity.
As will be understood by those skilled in the art, over time ambient temperature changes will effectively alter the optical length of the laser or resonator cavity even though the laser or resonator cavity may be mounted on super-invar bars. Typically, the drift due to changes in the air temperature is approximately 300 MHz/° C. and the drift to changes in the laser cooling water temperature is approximately 600 MHz/° C. The Free Spectral Range (“FSR”) of a laser is typically approximately 180 MHz and hence as will be understood by those skilled in the art and as will be made apparent in the following description, the laser only needs to drift by approximately half of the FSR (i.e. approximately 90 MHz) for the laser to change from operating in SLM to operating in a dual lasing mode. This represents a temperature change of only approximately 0.1° C. The output of the laser will therefore begin to drift in frequency over a period of time. In particular, the relative laser frequency will begin to vary with respect to the resonance frequency of the etalon.
FIGS. 1A and 1B show the typical pulse energy and the transmission of an etalon as the frequency drifts. It will be understood by those skilled in the art that because of the intrinsic transmission of the etalon (Airy function with peaks at resonances) the laser losses will also vary with respect to the laser frequency relative to the etalon resonance frequency. The laser may not therefore always be in SLM which can be particularly disadvantageous especially in certain applications such as holography. The simultaneous oscillation of two longitudinal modes will therefore occur when these two modes undergo substantially the same losses.
FIG. 2A shows a mode of operation wherein one longitudinal mode is clearly less lossy than other modes and hence the laser will operate in SLM. FIG. 2B shows the situation when two longitudinal modes experience substantially the same losses. In this situation, both laser modes will oscillate substantially simultaneously and hence the laser will no longer operate in the desired SLM mode of operation. The optical length of the laser is equal to qλ/2, wherein q is the longitudinal index of the operating mode.
In order to keep the laser operating in a SLM the laser needs to be stabilised. However, measuring the absolute value of the laser frequency in order to stabilise the laser is largely impractical for various reasons.
It is known to introduce a defect or a marker into the energy profile of a laser in order to assist in stabilising the laser. For example, in inhomogeneously-broadened gas lasers the Lamp dip may be used as a marker of the line center if it is deep enough. If not, then the gain curve itself may be used.
It is also known to introduce a saturable absorber inside a laser cavity and to use it as a reference. The saturable absorber is resonant at the operating wavelength. The defect in the profile is then a peak whose bandwidth is normally narrower (i.e. more accurate for modulation) than the Lamb dip.
However, it is generally disadvantageous to have to introduce a defect or marker into the energy profile of a laser, especially a solid state laser. Moreover, the broadening in a solid state laser is homogeneous.
It is therefore desired to stabilise the energy of a laser, especially a solid state laser, without needing to introduce a defect or marker and without, for example, having to provide a special cell including a saturable absorber.